Monopulse receivers are known in the art. Typically these are used in radar systems in which a continuous rf signal is generated and a reflected return from a moving target is detected within the antenna mixer, oscillator radar system. The monopulse antenna receiver obtains a sense of directivity for the target.
When a monopulse transceiver is needed at high frequencies such as of the order of 24 gigahertz high efficiency and low losses are needed to provide an effective low noise receiver.
In a copending patent application entitled, "Microstrip Antenna" and filed in the U.S. Patent Office as of Jun. 29, 1994 bearing Ser. No. 08/267,586 and assigned to the same assignee, a high efficiency antenna is described which is particularly effective in a radar vehicle collision avoidance system.
With a dual channel monopulse receiver in accordance with the invention, a low loss receiver is obtained that can be particularly effective for use in a vehicle collision avoidance system operating at very high frequencies of the order of 24 gigahertz.
This is achieved by employing in one form of the invention a planar microstrip antenna of the type as described in the aforementioned copending patent application. In such antenna, a main array is broken in two parts to create a left- and right-located arrays of patch-radiating elements. These are located on a substrate placed on one side of and connected by lattices of traces on the substrate. The respective lattices are connected to feed points which extend through the antenna plate to a substantially planar microstrip transceiver circuit located on the other opposite side of the plate.
The microstrip circuit includes an rf oscillator, a pair of microstrip rf mixers a reactive divider and a 180.degree. microstrip hybrid coupler connected between the antenna feedpoints and the rf mixers. RF power from the oscillator is efficiently coupled in a balanced manner to the respective antenna arrays and return signals are efficiently coupled from the arrays to the respective rf mixers with a microstrip sum and difference signal generator. This produces an amplitude sum of the rf return signals which is coupled to a sum channel rf mixer and also produces an amplitude difference of the return signals which is coupled to a difference channel rf mixer. The oscillator signal is applied to each of the respective mixers for combination with the respective sum and difference signals to produce sum and difference outputs which are also representative of doppler variations in the returns.
The hybrid mixer associated with the difference channel provides a large amount of isolation between the antenna and to the oscillator, of the order of 25 db, so that little oscillator signal can be coupled to the left and right antenna arrays via the difference channel path. As a result, during transmission only an in-phase oscillator signal is applied to both left and right arrays. The combined effect is a sharper beam during transmission while during reception, the difference signal generation results in a pair of closely-spaced split beams. The split beams and the single center beam from the effect of the sum channel can together yield directional information of a target.
With a transceiver/receiver in accordance with the invention the antenna produces beams, which can thus be used to derive, in addition to the presence of a target, an indication of its direction relative to the path of a vehicle on which the antenna is mounted. The receiver exhibits a high efficiency, excellent resolution and low losses for good target detection capability.
It is, therefore, an object of the invention to provide a microstrip transceiver capable of operating at millimeter wavelengths for use in the detection and direction of targets for vehicle collision avoidance. It is a further object of the invention to provide a directional millimeter wavelength antenna and microstrip transceiver using a planar array of patch elements and which exhibits low losses, a high efficiency and is economic to manufacture.
These and other objects and advantages of the invention can be understood from the following detailed description of an embodiment in accordance with the invention as illustrated in the drawings.